Centralisers perform important functions in wellbore operations. Centralisers may be used, for example, to ensure that a tubular or a portion of a tubular does not come into contact with a wellbore surface. This provides protection for the tubular against wear due to friction or impact with the borehole during run-in. A centraliser may be positioned on a tool string or completion string to provide stand-off protection to part of the string that is particularly sensitive to wear, friction or impact with the bore wall. This includes tool joints, sandscreens, and flow control devices.
Centralisers also have an important function in cementing applications. A poorly centralised tubular can lead to a poor fluid sweep of drill cuttings prior to cementing and the failure to form a cement bond around the entire circumference of the annular space between the tubular and the wellbore. This can result in poor isolation of well fluids, which can ultimately lead to uncontrollable flow of well fluids to the surface or to subterranean geological formations.
Centralisers are provided with blades or other formations to create stand-off from the body, to provide a large flow bypass area, and to assist with creating a turbulent flow of mud and cement. However, micro-channels may still be formed between the cement and the bore wall and/or between the outer surface of the centraliser body or blades and the bore wall.
A well packer provides a seal in an annulus formed between an exterior surface of a tubular and an interior surface of well casing or a wellbore. Known forms of well packers are introduced in an unexpanded condition to the downhole environment in which they are to be used and expanded in-situ to provide the desired seal. In one form, the well packer expands upon coming into contact with a well fluid. In another form, the well packer comprises movable parts that are actuated in-situ to form the seal.
The integrity of the annular seal created by a well packer is paramount. It is advantageous for the tubular on which the packer is located to be centrally located in the bore, such that, when the packer is expanded, it exerts a force against the bore that is substantially uniformly distributed around the circumference. If the tubular is positioned to one side of the bore, which is typically true for an inclined bore, the expansion force of the packer will have to act against the side load weight of the tubular to move to its expanded condition. If the expansion force is insufficient to overcome the side load weight, the packer may seal asymmetrically in the bore, with the packer having a radially short side (on the low side of the bore) and a radially longer side (on the high side of the bore). This results in a potential failure mode between the packer and the bore wall on the high side of the bore.